In general, sintered products made by powder metallurgy are advantageous in cost over ingot steels obtained through forging and rolling steps and has wide utility as parts of motor vehicles and office automation apparatus. However, the sintered product has pores which are inevitably formed during the course of its fabrication. These remaining pores of the sintered powder-metallurgical materials impairs the mechanical properties of the materials, as compared with completely dense materials. This is a result of the pores acting as stress concentrations and also because the pores reduce the effective volume under stress. Thus, strength, ductility, fatigue strength, macro-hardness etc. in iron-based powder-metallurgical materials decrease as the porosity increases. Impact energy is, however, the property the most adversely affected.
Despite their impaired impact energy, iron-based powder-metallurgical materials are, to a certain extent, used in components requiring high impact energy. Naturally, this necessitates high precision when manufacturing the components, the effect of the porosity on impact energy being well-known.
The impact energy of sintered steel may be increased by alloying with Ni, which augments the strength and ductility of the material and, furthermore, causes shrinkage of the material, i.e. a density increase. There is, however, an increasing demand for powders which do not contain nickel since, inter alia, nickel is expensive, gives dusting problems during the processing of the powder, and causes allergic reactions in minor amounts. From an environmental point of view, the use of nickel should thus be avoided
Sintered components having high impact strength without using Ni as alloying element are disclosed in U.S. Pat. No. 5,728,238. This patent discloses that it is possible to obtain impact strength of up to 100 J by using an iron-based powder which, in addition to Fe, contains Mo and P, and in which the content of other alloying elements is maintained on a low level. This material is, inter alia, characterised by the fact that sintering even below 1150.degree. C. results in an impact energy which is higher than that of powder-metallurgical materials sintered at higher temperatures. Further, the material has excellent compressibility and is capable of considerable shrinkage, giving a sintered material of high density. For one and the same density, this known material has a substantially higher impact energy than today's powder-metallurgical materials. A serious restriction, however, is that these sintered products have relatively low tensile strength of about 430 MPa.